The importance of a serine at the inner mouth of the channel (end of domain IV S6 segment) was studied. Its mutation to alanine abrogated the ability of the channel to enter into mode 2 gating upon stimulation by activation of PKA or addition of BAY-K8644. The serine (Ser 1517) may be a key regulatory site that determines, in coordination with the previously studied Ser 1142 in the selectivity filter, the responsivenes of the channel to phosphorylation and the dihydropyridine agonist. We are testing the hypothesis that loss of mode 2 gating is due to establshment of an ionic bridge between Ser 1517 and one of two nearby lysines from domain I, thus "freezing" the channel in mode 1 gating.